Which field to study to learn & create a.i generated simulations?
You need to define "simulation" more specific. Playing Mario, Swapping face on image/video, or generating simulation of objects that are orbiting use different techniques.


Playing Mario or "AI that playing game": the AI agent trained on available environment (Mario game, so the environment is not generated) and learn the best sequential actions to achieve the goal. It runs the game thousand times, when it did a wrong action then it gets "penalties" that improve its knowledge. The algorithm that can be used is Reinforcement Learning, but some earlier paper use Genetic Algorithm to generate the best action
Face swap: It's close to computer vision area, some methods that I know use Style Transfer principle (Convolutional Neural Network) to make transformation of face of one image to another image. You can read the basic of style transfer here.
Generating physical movement: I don't know too much about this topic but I know there are some papers talk about this, Fluid Net from Google workers and this paper from TU Munchen. At a glance they also use CNN to improve the result but the main simulation came from Euler Fluid Equation. So if you need to generate object that orbiting, I think you need to find equations that models that movement.


Hope it helps!